An electrostatographic printing machine such as a photocopier, laser printer, facsimile machine or the like employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image is transferred to a support material such as paper either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse the image to the support material.
Many types of developer compositions, including both dry developer compositions and liquid developer compositions, have been proposed for use in the development of latent electrostatic images.
In liquid developer compositions, various types of colorant and dye are attached to a resin dispersed in an insulating liquid carrier. In the so-called dry type developing method, colorant is dispersed in a fine powder.
Conventionally, toner for developing electrostatically charged images may be produced by melt-mixing the soft polymer and pigment whereby the pigment is dispersed in the polymer. The polymer having the colorant dispersed therein is then pulverized.
Normally, pigments are created in a chemical reaction in an aqueous phase. The pigment particles are filtered and washed. In the pigment manufacturing process, an aqueous slurry called a wet cake may be produced which is about 40% to about 85% by weight water, with about 50% by weight water being typical.
The amount of pigment necessary for the optimal color in a colored xerographic toner is critically dependent upon its dispersion within a toner; the better the dispersion, the higher the chroma for less pigment. Very good dispersions of colored pigments in a polymer matrix can be obtained by using a process called flushing. At present, some of the best dispersions are obtained by flushing the pigment into the resin to be used for making the toner.
Flushing is believed to involve the following process. The wet cake is put into a mixer to which is added a solution of a polymer such as Pliotone.RTM., a styrene-butadiene compound manufactured by Goodyear, and a solvent, such as toluene. The solution is gently mixed and the organic phase of the solution is adsorbed by the pigment. The water from the aqueous phase is displaced and poured off. The remaining solvent in the solution is removed by a vacuum as the solution continues to be mixed. The molten mass that results is comprised of about 50% pigment and about 50% polymer. The mass is cooled and crushed. Additional resin may be added, and all of the ingredients may be melt-mixed in an extruder.
This process is believed to work because in the wet cake form the pigment particles are kept isolated from one another by a layer of water surrounding them. The polymer solution displaces the water because the solution preferentially wets the pigment particles. When the water is separated it is simply poured off and the solvent is vacuum evaporated so that each particle is now surrounded by a film of polymer; hence a high quality dispersion is possible.
This process is very costly, as it requires the use of a solvent and a relatively large quantity of pigment for the correct chroma. It also requires a number of manufacturing steps and monitoring equipment.
However, if this process is not used and the pigments are simply allowed to dry or are force dried before mixing with resin, the particles agglomerate and are extremely difficult to separate, even in high shear melt mixing processes such as extrusion or in a mill.
U.S. Pat. No. 4,623,604 (Takagiwa et al.) discloses a triboelectric stabilized toner for developing electrically charged images comprising nuclear particles formed by polymerization, hydrophobic silica, and a resin. The resin is coated on the nuclear particles and the silica is dispersed in the resin. The resin is dissolved in a solvent. The solvent exerts a lesser solvating action against the nuclear particles than against the resin to form a solution. The hydrophobic silica in the solution is then dispersed, the nuclear particles are mixed in the dispersion and the mixture is dried. Drying is achieved by means of a process which may include air drying, decompression drying or spray drying by a spray drier. The solvent is removed and a toner with a resin coated layer is obtained.
The use of liquid pigment dispersions as colorants for paints and inks is also known. In U.S. Pat. No. 3,778,287 (Stansfield et al.) dispersions of inorganic pigments, lakes or toners in organic liquids containing polyesters dissolved therein having acid values up to 100 derived from certain hydroxy-containing, saturated or unsaturated aliphatic carboxylic acids are described. While liquid colorants offer the distinct advantage of being more readily incorporated into the medium to be colored than dry pigments, their commercial significance is seriously limited due to the problems of handling and storing potentially hazardous liquid chemicals. Thus, from an economic and safety standpoint, it is desirable to have the colorants in a dry, storage stable form which is readily dispersible in a wide variety of coating media without detriment to any of the desirable properties of coating produced therefrom.
U.S. Pat. No. 3,607,335 (Belde) discloses pigment dye formulations which contain the pigment dye dispersed in diglycol terephthalates or linear oligomers of terephthalic acid and ethylene glycol in plastic filaments or threads. The pigment formulations are manufactured in a batch process in which the pigment-dyes are flushed with polymer and kneaded from one to four hours.
U.S. Pat. No. 4,391,648 (Ferrill) discloses particulate pigment compositions which are readily dispersible in oleoresinous vehicle systems wherein the compositions comprise by weight from 25-95% pigment and 5-75% of water-insoluble, friable thermoplastic polyester resins. The pigments can be in the form of a dry lump, wet cake, or slurry of the pigments in an organic solvent. Press cakes are preferred to avoid aggregation that occurs during drying. The compositions of the invention can be formed in any convenient manner for intimately mixing a pigment with a normally solid resin, as for example, by ball milling in a dry state, pebble milling in an aqueous medium, high speed stirring in the presence of a solvent for the resin, and the like, and then removing any aqueous medium or solvent. Conveniently, the compositions are prepared by forming an aqueous dispersion of the pigment, preferably by stirring the pigment in water in the presence of a surfactant, adding the resin to the aqueous dispersion and agitating the aqueous pigment-resin slurry at a temperature above the softening temperature of the resin. No extruder is used to disperse the pigment in the resin.
U.S. Pat. No. 4,054,465 (Ziobrowski) discloses lead chromate-containing pigments having improved dispersibility, heat stability and resistance to abrasion in thermoplastic systems. The pigments comprise silica coated lead chromate-containing particles having absorbed on their surface from 1-15% based on the weight of the coated particles of certain liquid organopolysiloxanes. The improved lead chromate-containing pigments of this invention are produced by depositing on the lead chromate-containing particles at least one substantially continuous coating of dense amorphous silica, with or without alumina, or a solid glass-like alkali polysilicate, and contacting the coated particles with certain liquid organopolysiloxanes. Following application of the silica coating to the lead chromate particles, the coated particles are contacted with a liquid organopolysiloxane under conditions which do not effect substantial polymerization a curing of the polysiloxane. The coated pigment can be in the form of a ground dried lump, a wet cake, a slurry of the coated pigment in water, or an inert organic diluent. The mixture is ground or vigorously agitated at room temperature in a blender. The liquid polysiloxane can also be applied directly, as by sprinkling on the dry coated pigment and then grinding wetted pigment in a high speed grinding device.
In the examples given in the patent, the pigments, except for one of the control pigments, are pre-treated prior to being dry blended with commercial injection grade polystyrene granules and the blended sample is twice passed through a Sterling laboratory extruder. There is no indication that the untreated pigment is in the formula of a wet cake, nor is the invention directed towards an improved method of manufacturing toner.
U.S. Pat. No. 4,247,338 (Ziobrowski) discloses a metal chromate pigment composition, particularly a lead chromate composition, which exhibits low dusting characteristics and improved heat stability in thermoplastics. The pigment particles are treated with a combination of certain metal salts or fatty acids and plasticizers. The lead chromate particles with or without a silica or silica aluminum coating are in the form of a ground dried lump, wet cake, a slurry of the pigment in water or a suitable organic diluent. The pigment, fatty acid and plasticizer are mixed. In the examples given in the patent, the pigments, except for one of the control pigments, are pretreated prior to being dry blended with commercial polypropylene pellets, passing the blended sample thrice through an extruder. There is no indication that the untreated pigment is in the form of a wet cake, nor is the invention directed towards an improved method of manufacturing toner.
In U.S. Pat. No. 4,894,308 (Mahabadi et al.), a process for preparing an electrophotographic toner is disclosed which comprises premixing and extruding a pigment, a charge control additive and a resin. The pigment and the charge control additive may be premixed prior to being added to the extruder with the resin; alternatively, the pigment and charge control additive may be premixed by adding them to the extruder via an upstream supply means and extruding them, and subsequently adding the resin to the extruder via a downstream supply means. There is no mention of the use of pigment in the form of a wet cake.